Preparation of thin cermet films by radio frequency sputtering



Dec. 2, 1969 C. H. LANE 3,481,854

PREPARATION OF THIN CERMET FILMS BY RADIO FREQUENCY SPUTTERING FiledJan. 20, 1967 J0 I H 36 -w4+d 4/ 46 J5 aw;

an 26 air/mama 55 INVENTOR. czraeuzi/va' United States Patent 3,481,854PREPARATION OF THIN CERMET FILMS BY RADIO FREQUENCY SPUTTERING Clyde H.Lane, Rome, N.Y., assignor to the United States of America asrepresented by the Secretary of the Air Force Filed Jan. 20, 1967, Ser.No. 610,732 Int. Cl. C23c /00; B44d 1/02 US. Cl. 204-192 3 ClaimsABSTRACT OF THE DISCLOSURE The invention described herein may bemanufactured and used by or for the United States Government forgovernmental purposes without payment to me of any royalty thereon.

This invention relates to cathodic sputtering of metal and dielectricfilms. In a more particular aspect, this invention relates to animproved method for the cathodic sputtering of cermet type films withina plasma generated by a radio frequency field. Cermets are compositematerials consisting of an intimate mixture of ceramic and metalliccomponents, usually in the form of powders.

The ceramic component may consist of metallic oxides, carbides, borides,silicides, nitrides or mixtures of these compounds; while the metalliccomponent may include a wide variety of metals such as aluminum, cobalt,iron, chromium, silicon, silver, gold, platinum, palladium and others.The selection of the particular ceramic and metallic component dependson the contemplated application of the respective cermet. Frictionparts, cutting and drilling tools, as well as nuclear reactor fuelelements have been made from cermets. Generally, cermets possesscorrosion and high temperature resistance, together with high strengthand high wear resistance. Heretofore, commercial cermet type glazes havebeen coated onto various substrates by using conventional silk screentechniques. Such techniques produce a relatively thick film which doesnot fined utility, for example, in the production of silicon basedmicro-circuits.

With the present invention, however, it has been found that thin uniformcermet films can be deposited upon various substrates using the radiofrequency sputtering technique of this invention. The films produced bythe method of this invention find special utility in the production ofmicro-circuits for low power circuits and analog circuits.

The primary object of this invention is the provision of an improvedmethod for effecting the deposition of cermet materials onto asubstrate.

A further object of this invention is the provision of an improvedmethod for effecting the deposition of thin uniform layers of cermetmaterials upon a substrate.

Another object of this invention is the provision of an improved methodfor effecting the deposition of cermet films onto a substrate by meansof a sputtering technique.

Other objects, advantages and features of the invention will be pointedout in the following detailed description, taken in conjunction with theaccompanying drawing which depicts an apparatus suitable for use in thepractice of the method of this invention.

Patented Dec. 2, 1969 Referring to the drawing:

The figure is a schematic representation of a cathodic sputteringapparatus suitable for depositing thin films of cermet materials.

In its broad aspects the invention disclosed herein contemplates theproduction of thin uniform films of cermet materials onto a substratebase by maintaining the target substrate surface under controlled ionicbombardment during sputtering so as to eifect removal from said surfacea quantity of sputtered material which will condense on a substrate to agiven thickness.

More particularly, in the practice of this invention, deposition isaccomplished by placing the source for the material to be deposited as aseparate electrode in a low pressure, high density gas discharge plasmawith low background impurity pressure between the cathode and an anode.The electrode, which provides the cermet material to be deposited, andthe substrate are positioned opposite to each other and are subjected toionic bombardment to effect deposition of the cermet material onto thesubstrate surface. The method consists of maintaining a desired vacuumwithin a vacuum chamber which has positioned therein an anode and acathode. The target consists of a ceramic substrate upon which a cermetglaze has been fired. A tungsten filament is also disposed within thevacuum chamber and sufiicient bias is supplied to the filament to causeelectron emission. The filament is then biased cathodically so as tofunction as a thermionic cathode. The cermet coated target electrode isconnected to an RF generator. A desired frequency, subject to FCCcontrol, is applied to the target electrode While immersed in theplasma, thereby causing sputtering of the cermet material. A substrateplaced within the discharge will then pick up the sputtered cermetmaterial effecting deposition of the cermet onto the substrate surface.

Referring more particularly to the drawing, there is shown in the figurean apparatus suitable for depositing a thin uniform film of cermetmaterial through the method contemplated by this invention. Shown in thefigure is a vacuum chamber 10 in which are disposed a ceramic targetelectrode 12 and an anode 14. The target electrode 12 may be composed ofan electrically non-conductive base such as alumina, Vycor or quartz andis coated with a cermet glaze 16 which has been either sprayed, screenedor painted thereon and fired at the recommended temperature, usuallyabout 800 C. The cermet target electrode 12 is connected to a metalelectrode 18. A sheet of aluminum foil 20 is placed on the back of theceramic cathode and is held there by a backer plate 22 of the samematerial that is used as the substrate of the target electrode 12. A tabof aluminum 24 is allowed to extend from this sandwich. Through this tabthe radio frequency energy from an RF generator 26 is fed to the targetelectrode 12, thereby causing sputtering of the cermet glaze 16 onto adesired substrate 28. Suitable substrates for use in this invention areglazed alumina or passivated silicon. The RF generator 26 is positionedoutside the vacuum chamber 10 but is connected thereto in a sealedrelationship by rubber gasket 30. Opposite the anode 14 is a thermioniccathode comprising a tungsten filament 32. A platform 34 ofnon-conductive material is disposed in a sealed relationship forsupporting the vacuum chamber 10. The vacuum chamber 10 may be a belljar or some other suitable container. Provision is made for evacuatingthe chamber by means of a conduit 36 which is connected to a suitablevacuum pump, not shown. Provision is also made for admitting to thechamber 10 a mixture of argon and oxygen or other gases through conduit38 during the sputtering process. The anode 14 which is electricallyinsulated by rubber gaskets 42 is biased by an electrical source 44. Thesubstrate 28 which is to receive the cermet glaze is supported adjacentthe side of the bell jar 10 by conventional supporting means as shown at40. The tungsten filament 32 is connected to a source of electricalenergy 46.

In the operation of the method of this invention, the vacuum chamber 10is first evacuated, flushed with an inert gas as, for example, any ofthe rare gas family, such as helium, argon or neon, and the chamber thenreevacuated. The system is now pumped down to 1x10" torr and backfilledwith the sputtering ambient. An argonoxygen gas mixture is used if thecermet has a metal oxide basis to a pressure of about l5.0 1O- torr.Again, the system is pumped down but only to 5 10 torr if a glowdischarge is to be used or to 5X10- if a low energy electron dischargeis used. A small positive bias of about 40 volts is placed on the anode14. The tungsten filament 32 is positioned opposite the anode 14 andsuflicient bias is applied to cause electron emission. The targetelectrode 12, consisting of the ceramic substrate to which the cermetglaze has been fired on, is placed within the plasma generated by theslow electrons at 1-10 microns of mercury pressure. The target electrode12 is backed by the metal electrode 18 which is shielded from the plasmaand is connected to an RF generator 26 of about 1 kilowatt. A frequencyof about 40 mc. (subject to FCC control) is applied to the cathode 12while immersed in the plasma, thereby causing sputtering of the cermetmaterial. A substrate 28 placed within the discharge will then pick upthis sputtered material.

For the method of this invention, it is preferred to employ a sputteringsource anode bias of about 40 volts, a pressure of from 5X10" torr to5x10 torr and a frequency energy of about 40 mc. The spacing between theanode and the cathode is not critical. I

For effective operation the tungsten filament should be shielded fromthe target to some extent in a low energy electron discharge which mustbe present for sputtering to occur. Balancing of various factors such asvoltage, pressure and relative positions of the target, anode andsubstrate to obtain a high quality deposit is well known in thesputtering art.

An example of the present invention is described below. By employingproper voltage, pressure and spacing of the various elements within thevacuum chamber, a thin uniform film of an oxidized palladium-gold cermetwith a ratio of 45 percent palladium and 55 percent gold can bedeposited upon glazed alumina substrate. Sputtering is conducted for aspecified time at a voltage of 40 volts and a pressure of 5 X torr usingan oxygenargon gas mixture with the substrate positioned about fourinches from the target.

The thickness of the palladium-gold cermet deposited on the substratewas within the range of from 500 to 1000 angstroms.

The sputtering target electrode of the example was composed of analumina substrate on which had been fired the palladium-gold glazereferred to above. The palladiumgold glaze is either sprayed, screenedor painted onto the alumina electrode and fired at a temperature ofabout 800 C. These cermets possess a resistivity of from 1 to 20,000ohms per square per mil. The palladium-gold cermet used in the examplewas Glaze No. 6910. A frequency of about 40 mc. was applied to thesputtering cathode by means of the RF generator. Sputtering wasconducted for a period of about one-half to one hour producing the thinuniform cermet film referred to above with a thickness of approximately500 angstroms.

Circuitry for the operation of the low pressure plasma discharge systemof the invention, as shown in the figure, and referred to in the exampleis well known in the art. The example and the apparatus described in thefigure are included merely to aid in the understanding of the invention.Obviously, variations may be made by those skilled in the art withoutdeparting from the spirit of the invention, the scope of which isdefined by the appended claims.

While the invention has been described in detail in the foregoingspecification and the drawing similarly illustrates the same, theaforesaid is by way of illustration only and is not restricted incharacter. The several modifications which will readily show themselvesto persons skilled in the art are also considered in the scope of thisinvention With reference being made to the appended claims.

What is claimed is:

1. A method for the deposition of a film of cermet material onto awork-piece surface comprising the steps of forming a cermet coatedtarget electrode, including providing an electrically non-conductiverefractory base, coating said base with a mixture of a ceramic componentand a metallic component and firing said mixture at a temperaturesufiiciently high to form a coating of said cermet material positioningan anode, a thermionic cathode, and said target electrode within avacuum chamber in a spaced relationship to each other, supporting saidcermet coated target electrode and a work-piece within the vacuumchamber oppositely disposed to each other in a spaced relationship,introducing a low pressure ionizable gas atmosphere into the vacuumchamber, applying a potential to the said thermionic electrode to causeslow flow of electrons from said thermionic cathode to said anodethereby creating a low pressure gas discharge plasma within said vacuumchamber, and applying radio frequency energy to the cermet coated targetelectrode to effect sputtering of the cermet material onto the surfaceof the work-piece.

2. A method in accordance with claim 1 wherein said radio frequencyenergy is applied at about 40 megacycles.

3. A method in accordance with claim 1 wherein said atmosphere possessesa pressure of between one to ten microns of mercury.

No references cited.

ROBERT K. MIHALEK, Primary Examiner US. Cl. X.R. 106-55

